Lithium ion batteries capable of intercalating and deintercalating a lithium ion have recently been put to practical use as a nonaqueous secondary battery having high discharge potential and discharge capacity. A positive electrode active material or negative electrode material capable of intercalating and deintercalating lithium is dispersed in a disperse medium together with a (electric) conducting agent and a binder by means of a dispersing machine, such as a homogenizer or a planetary mixer, to form a dispersion paste. The dispersion paste is applied onto a current collector and dried to prepare a sheet electrode. A nonaqueous solvent is frequently selected as a disperse medium so as to avoid direct deterioration of the active material by water and deterioration of battery performance by a residual water content in battery assembly. However, use of an organic solvent incurs such problems as deterioration of the working environment, environmental pollution arising from the waste, and difficulty in forming a uniform film. Hence, it has been demanded to handle electrode active materials in a water-dispersion system.
It has been pointed out that a positive electrode active material or a negative electrode material, which is a compound capable of intercalating or deintercalating lithium, especially a negative electrode material is not stable in water and gives adverse influences to the discharge capacity and charge and discharge cycle characteristics of a nonaqueous secondary battery. JP-A-1-296567 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-3-145071, and JP-A-3-64860 disclose methods comprising previously washing an active material with water. These methods are still unsatisfactory in improving discharge characteristics. While the reasons why charge and discharge cycle characteristics are deteriorated where an electrode material mixture paste of a water-dispersion system is used have not yet been elucidated, it has been suggested that there is some factor which increases the internal resistance of a battery in charge and discharge cycles in addition to the above-mentioned reasons.
An object of the present invention is to provide a nonaqueous secondary battery having improved charge and discharge characteristics such as discharge potential, discharge capacity, and charge and discharge cycle life, and a process for producing the same. Another object of the invention is to provide a nonaqueous secondary battery having an electrode prepared by using a water-dispersion electrode material mixture paste having improved dispersibility and a process for producing the same. A further object of the invention is to provide a process for preparing a water-dispersion paste of an electrode material mixture with improved dispersibility and a nonaqueous secondary battery which is produced by taking advantage of that process. A still further object of the invention is to provide a nonaqueous secondary battery having the optimum thickness which is prepared by using an electrode material mixture having improved dispersibility and a process for producing the same.